Due to the behavior of a discharge lamp (e.g. fluorescent lamp), it needs a high voltage, which is much higher than the normal operational voltage and is referred to as start voltage, to break down the gas therein so as to produce a glow discharge. Then the lamp transits to the phase of burning discharge during normal operation. Such behavior requires that the resonant operation circuit should produce a high enough start voltage so as to ignite the fluorescent lamp. Meanwhile, such start voltage should be limited within a certain magnitude in compliance with certain requirements like safety specification etc.
Patent document US 2006/0181226A1 discloses a circuit arrangement for operation of a gas discharge lamp, which has a free-running half-bridge inverter. A stop device St is used in said arrangement to enable the drive to the half-bridge switches only during an on time. The oscillation frequency of the half-bridge inverter can be reliably adjusted by the duration of the on time, and this allows control of the lamp operating variables. In FIG. 1 of this document, the threshold value device Sc compares the current in switch S2 with a predetermined reference value. If the reference value is exceeded, the threshold value device Sc sends an interrupt signal. However, said document does not teach the processing of the following driving period of the other switch S1. In addition, in order to adjust the on time, the circuit taught in said document is very complicated and has a high cost.
Patent document U.S. Pat. No. 7,053,561, B2 also discloses a circuit for operating a discharge lamp, wherein the current through the components of a half-bridge of an electronic ballast is intended to be limited during the starting phase. For this purpose, the gate of a half-bridge transistor T2 is driven by a current limiting circuit (D1, D2, D3, T3, C3) in such a way that the current through the transistors T1, T2 is limited during the starting phase, whereas it remains unlimited during the glow phase and burning phase of the discharge lamp LA. The gate of transistor T2 is controlled such that the current flowing through T2 does not exceed a certain threshold value. During the starting phase of the lamp, the transistor T2 is switched off early, if appropriate, in each switching cycle. But said document does not teach the processing of the following driving period of the other switch T1, either.